In metal fabrication processes solvent extraction is generally used, by means of which metals can be extracted from aqueous solutions using organic extraction solutions. It was previously known that gold could be extracted from a solution of hydrochloric acid using some ketone such as methyl butyl ketone, phosphate such as tributyl phosphate, alkyl amine, and long-chained ether such as diethyl glycol dibutyl ether as extractant. The last-mentioned is also known by the abbreviation BUTEX and by the name dibutyl carbitol or DBC, and is in use in industrial processes. The book “The chemistry of gold extraction” (reference 1) provides a good overview of current known extraction methods.
Gold can be extracted very effectively with DBC, but it has been found to have several negative properties, which are described in reference (1). In industrial processes, such as the Hoffmann process (2), it has been observed that the DBC phase settles very slowly from the aqueous phase, which has complicated both the extraction stage and the scrubbing stage of the extraction phase. In addition, the solubility of DBC in water is extremely high (approx. 3 g/L, 25° C.), leading to reagent losses in the extraction and scrubbing stages. The flash point of DBC is also relatively low. Since DBC binds gold so strongly, the gold has to be reduced directly from the organic phase. Generally, reduction is performed at an elevated temperature using an oxalate. Along with the reduced gold precipitate, a loss of expensive extractant is also generated. In addition, the unpleasant smell of DBC requires special arrangements regarding the working environment.
It is known in the literature (3) that long-chain alcohols also extract gold. Among others, Grant (4) has presented and shown empirically that DBC could be replaced with long-chain alcohols. WO application publication 2009105832 is based on the use of the same long-chain alcohols for gold extraction from a solution of hydrochloric acid.
When using long-chain alcohols as the extractant, an aliphatic hydrocarbon mixture is used as diluent in the above-mentioned article by Grant and in the WO application publication. However, it is known that the hydrocarbon diluents in question may oxidise in process conditions, whereupon surface-active long-chain carboxyl acids etc. are generated. This change of the solvent is unfavourable for the extraction process, since it impairs settling and may increase the extraction of impurity metals.